1. Field of the Invention
The present invention relates to switching mode power converter circuits, and more particularly, to a switching mode power converter having a front end regulator to control input supply ripple.
2. Description of Related Art
Switching mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switching mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is a particular type of switching mode converter in which an input DC voltage is coupled in series with a periodically driven switch to produce a rectangular waveform at a phase node. The rectangular waveform is rectified by a diode. An output filter including an inductor and capacitor produces a smoothed DC output voltage from the rectified rectangular waveform.
More particularly, the buck converter chops the input DC voltage to achieve an output DC voltage that is lower than the input DC voltage. The input supply current approximates a square wave oscillating at the chopping frequency, the peak of which is controlled by the output load current. Input supply voltage ripple is formed by the current waveform passing through the equivalent series resistance (ESR) of the input supply decoupling capacitor and the copper trace impedance at the chopping frequency, and the equivalent series inductance (ESL) of the input supply decoupling capacitor and the copper trace inductance due to the rate of change of the current signal (dl/dtr). It is desirable to attenuate the input supply voltage ripple to prevent it from propagating to other integrated circuits powered from the same supply.
One way to attenuate the ripple is to include an RC (resistor capacitor) based low-pass filter to attenuate the switching noise produced by the integrated circuits of the system supply. A drawback of this approach is that a resistor based system would have a load dependant dropout the magnitude of which would increase with load current; i.e., the higher the voltage drop, the worse the efficiency. Another way to attenuate the ripple is to include a high value, lower ESR specialist capacitor from the system supply to ground to decouple the switching noise from the system supply. A drawback of this approach is that very low ESR capacitors generally have a cost premium associated with them.
Accordingly, it would be desirable to provide an improved way to control input supply ripple in a switching mode power converter.